1. Field of the Invention
This invention relates to an optical interferometric contact-free measuring method for sensing, by use of an optical laser beam, motional surface deformation (vibration displacement) of a workpiece wherein ultrasonic energy is applied to the workpiece to cause the vibration thereof.
2. Description of the Prior Art
Ultrasonic testing utilizing a piezoelectric element, such as a crystal transducer or PZT (PbZr.sub.1-y Ti.sub.y O.sub.3), is known as a nondestructive means for detecting internal faults or cracks of steel materials, or internal faults of welded joints thereof. In this kind of testing, however, a detector must be placed in direct contact with the workpiece to be tested or the detection must be effected through a medium, such as water or oil, for assisting the transmission and reception of ultrasonic waves between the detector and the workpiece. When the workpiece is of a high temperature, it is very difficult to protect the detector from damage in the first case or to achieve exact detection since the propagation of ultrasonic waves is disturbed due to the evaporated medium in the second. Furthermore, when the workpiece moves at a high speed, it is difficult to support the detector to move it at the same speed as the workpiece.
It is known to measure vibration displacement or surface deformation of a workpiece by an optical laser beam without physical contact with the workpiece. In this method, a coherent measuring laser beam is applied incident upon a workpiece surface and the phase difference between a reference wave and a wave of laser beam light reflected from the workpiece surface is measured by an optical heterodyne or homodyne measurement method. The phase difference corresponds to surface deformation, irregularity, of displacement of the workpiece surface. The concept for optical homodyne measurement of fine vibration displacement is disclosed in, for example, "Keisokuron" (Measurement) by K. Iijima and Y. Tsuzuki, page 121 to 127, published in 1978.
One example of the prior art is the Michelson-type interferometer for measuring the motional surface deformation of a workpiece caused by subjecting the workpiece to ultrasonic waves. In this interferometer, coherent light produced by an optical laser beam source is split by means of a beam splitter, into two coherent light beams, i.e., a measuring beam and a reference beam. The measuring beam is applied incident upon the vibrating surface of a workpiece and the reference beam is applied upon a fixed reflecting mirror. The vibration of the workpiece can be caused by, for example, ultrasonic energy applied thereto. After reflection, both beams are combined by the beam splitter and caused to interfere on an optical detector. The vibrative motion of the workpiece surface changes the phase of the reflected measuring beam. On the other hand, no change occurs in the phase of the reflected reference beam. Therefore, a phase difference occurs between the two reflected beams.
In this prior art arrangement, several problems occur, particularly when the workpiece is, for example, a steel plate of a high temperature moving on a hot rolling line in a steelmaking process. First, the air on and above the workpiece surface flickers due to the heat of the workpiece, changing the refractive index of the beam. This modulates the measuring beam and forms an interferometric beat together with the reference beam in the optical detector, resulting in noise.
Second, since the workpiece moves, for example, on rolls, it vibrates in directions perpendicular to the movement thereof. That is, when the workpiece moves in the horizontal direction it oscillates in the vertical direction. Therefore, the motional surface deformation of the workpiece surface is caused not only by ultrasonic wave but also the workpiece's own movement. As a result of this, the measuring beam is modulated by two kinds of motional surface deformations. This also results in noise.